Communication protocols are often described in terms of a stack with well-defined interfaces between the layers. For example, the OSI reference model (and the TCP/IP architecture) defines seven protocol layers: application, presentation, session, transport, network, link, and physical. A system having the OSI seven-layer architecture (or the TCP/IP architecture) typically includes application layer circuitry, presentation layer circuitry, session layer circuitry, transport layer circuitry, network layer circuitry, link layer circuitry, and physical layer circuitry.
Line coding techniques are often employed in serial, digital transmission (at the physical layer) to transform the frequency spectrum of the transmitted data stream to adapt to channel characteristics and facilitate clock/data recovery by receiver systems. This is typically accomplished using a block code which encodes a number of user data bits as a larger number of bits, according to some pre-determined table or function. The added bits of overhead typically ensure a high transition density to facilitate clock/data recovery, and accomplish DC balancing to maintain running disparity near zero, which enables the use of AC-coupling. Since the code space is larger than the data space, a modest error detection capability is also afforded.
Various serial links for transmitting data and clock signals over various media (including but not limited to fiber optics and conductors) are well known. One conventional serial link, used primarily for high-speed transmission of video data from a host processor (e.g., a personal computer) to a monitor, is known as a transition minimized differential signaling interface (“TMDS” link). The characteristics of a TMDS link include the following:
1. video data are encoded and then transmitted as encoded words (each 8-bit word of digital video data is converted to an encoded 10-bit word before transmission);                a. the encoding determines a set of “in-band” words and a set of “out-of-band” words (the encoder can generate only “in-band” words in response to video data, although it can generate “out-of-band” words in response to control or sync signals. Each in-band word is an encoded word resulting from encoding of one input video data word. All words transmitted over the link that are not in-band words are “out-of-band” words);        b. the encoding of video data is performed such that the in-band words are transition minimized (a sequence of in-band words has a reduced or minimized number of transitions);        c. the encoding of video data is performed such that the in-band words are DC balanced (the encoding prevents each transmitted voltage waveform that is employed to transmit a sequence of in-band words from deviating by more than a predetermined threshold value from a reference potential. Specifically, the tenth bit of each “in-band” word indicates whether eight of the other nine bits thereof have been inverted during the encoding process to correct for an imbalance between running counts of ones and zeroes in the stream of previously encoded data bits);        
2. the encoded video data and a video clock signal are transmitted as differential signals (the video clock and encoded video data are transmitted as differential signals over conductor pairs);
3. three conductor pairs are employed to transmit the encoded video, and a fourth conductor pair is employed to transmit the video clock signal; and
4. signal transmission occurs in one direction, from a transmitter (typically associated with a desktop or portable computer, or other host) to a receiver (typically an element of a monitor or other display device).
A use of the TMDS serial link is the “Digital Visual Interface” interface (“DVI” link) adopted by the Digital Display Working Group. A DVI link can be implemented to include two TMDS links (which share a common conductor pair for transmitting a video clock signal) or one TMDS link, as well as additional control lines between the transmitter and receiver.
A typical DVI link includes a transmitter, a receiver, and a cable between the transmitter and receiver. The transmitter includes three encoder/serializer circuits, and the receiver includes three decoder circuits. The cable includes a conductor pair (Channel 0) for transmitting a stream of serialized data from a first encoder/serializer to a first decoder, a conductor pair (Channel 1) for transmitting another stream of serialized data from a second encoder/serializer to a second decoder, a conductor pair (Channel 2) for transmitting another stream of serialized data from a third encoder/serializer to a third decoder, and a conductor pair (Channel C) for transmitting a video clock from the transmitter to the receiver. The cable also include wires for a Display Data Channel (“DDC”) channel (which can be used for bidirectional I2C communication between the transmitter and receiver), a Hot Plug Detect (HPD) line (on which a monitor associated with the receiver transmits a signal that enables a processor associated with the transmitter to identify the monitor's presence), “Analog” lines for analog video transmission from the transmitter to the receiver, and “Power” lines for provision of power from the transmitter to the receiver.
In the transmitter, one encoder/serializer encodes data to be transmitted over Channel 0 (and serializes the encoded bits), another encoder/serializer encodes data to be transmitted over Channel 1 (and serializes the encoded bits), and a third encoder/serializer encodes data to be transmitted over Channel 2 (and serializes the encoded bits). Each encoder/serializer responds to a control signal (an active high binary control signal referred to as a “data enable” or “DE” signal) by selectively encoding either digital video words (in response to DE having a high value) or a control or synchronization signal pair (in response to DE having a low value). Each encoder/serializer receives a different pair of control or synchronization signals: one receives horizontal and vertical synchronization signals (HSYNC and VSYNC); another receives control bits CTL0 and CTL1; and a third receives control bits CTL2 and CTL3. Thus, each encoder/serializer generates in-band words indicative of video data (in response to DE having a high value), the first encoder/serializer generates out-of-band words indicative of the values of HSYNC and VSYNC (in response to DE having a low value), the second encoder/serializer generates out-of-band words indicative of the values of CTL0 and CTL1 (in response to DE having a low value), and the third encoder/serializer generates out-of-band words indicative of the values of CTL2 and CTL3 (in response to DE having a low value). In response to DE having a low value, each of the second and third encoder/serializers generates one of four specific out-of-band words indicative of the values 00, 01, 10, or 11, respectively, of control bits CTL0 and CTL1 (or CTL2 and CTL3).
Another serial link is the “High Definition Multimedia Interface” interface (“HDMI” link) developed by Silicon Image, Inc., Matsushita Electric, Royal Philips Electronics, Sony Corporation, Thomson Multimedia, Toshiba Corporation, and Hitachi. It has been proposed to transmit encrypted video and audio data over an HDMI link.
Another serial link (sometimes referred to as a “SATA” link) complies with the standard known as Serial ATA, Revision 1.0, adopted on Aug. 29, 2001, by the Serial ATA Working Group, for communication between a host and storage device. A host can be coupled to each of one or more storage devices, with a SATA link between the host and each storage device.
Other serial links differ from TMDS links by encoding data as N-bit code words that are not 10-bit TMDS code words, or by transmitting encoded video over more than three or less than three conductor pairs.
The term “transmitter” is used herein in a broad sense to denote any device capable of encoding data and transmitting the encoded data over a serial link (and optionally also performing additional functions, which can include encrypting the data to be transmitted and other operations related to encoding, transmission, or encryption of the data). The term “receiver” is used herein in a broad sense to denote any device capable of receiving and decoding data that has been transmitted over a serial link (and optionally also performing additional functions, which can include decrypting the received data and other operations related to decoding, reception, or decryption of the received data). For example, the term transmitter can denote a transceiver that performs the functions of a receiver as well as the functions of a transmitter. For another example, in a system including two transceivers which communicate via a serial link, each transceiver can be both a receiver and a transmitter.
Some serial links encode input video data (and other data) to be transmitted into encoded words comprising more bits than the incoming data using a coding algorithm other than the specific algorithm used in a TMDS link, and transmit the encoded video data as in-band characters and the other encoded data as out-of-band characters. The characters need not be classified as in-band or out-of-band characters based according to whether they satisfy transition minimization and DC balance criteria. Rather, other classification criteria could be used. An example of an encoding algorithm, other than that used in a TMDS link but which is used in some serial links, is IBM 8b10b coding. The classification (between in-band and out-of-band characters) need not be based on just a high or low number of transitions. For example, the number of transitions of each of the in-band and out-of-band characters could (in some embodiments) be in a single range (e.g., a middle range defined by a minimum and a maximum number of transitions).
The data transmitted between the transmitter and receiver of a serial link can, but need not, be transmitted differentially (over a pair of conductors). Also, although a TMDS link has four differential pairs (in the single pixel version), three for video data and the other for a video clock, other serial links include a different number of conductors or conductor pairs.
Typically, the primary data transmitted by a TMDS link are video data. What is often significant about this is that the video data are not continuous, and instead have blanking intervals. These blanking intervals provide an opportunity for auxiliary data to be transported, and they represent unused bandwidth. However, many serial links do not transmit data having blanking intervals. For example, audio serial links would typically transmit continuous data.
The term “stream” of data, as used herein, denotes that all the data are of the same type and are transmitted with the same clock frequency. The term “channel,” as used herein, refers to that portion of a serial link that is employed to transmit data serially (e.g., a particular conductor or conductor pair between a transmitter and receiver over which the data are transmitted, and specific circuitry within the transmitter and/or receiver used for transmitting and/or recovery of the data), where “data” is used in a broad sense to denote any content (e.g., application data, a clock, control information, or other content), and to the technique employed to transmit the data over the link.